Heathrow Airport should be closed and turned into a garden city for 200,000 people, London mayor Boris Johnson said as he put forward a plan to build a four-runway mega-airport in Kent within 20 years.

Under Mr Johnson’s plan, a £50 billion (€57.8 billion) project designed by architect Norman Foster and funded by the British government and private industry would come into operation by 2029 and serve 180 million passengers a year.

Heathrow is struggling to operate with two runways, though its owners yesterday reacted bitterly to Mr Johnson’s plans, saying it was “extraordinary” that he was talking about closing an airport that employs almost 120,000 people.

However, Mr Johnson insisted Heathrow would not be able to compete with newly developed airports elsewhere in the world with just two runways, while planning objections meant it was unlikely to get permission to expand, even if that was desirable.

“Those cities have moved Heaven and Earth to locate their airports away from their major centres of population, in areas where they have been able to build airports with four runways or more,” he said.

“We have to build an airport capable of emulating that scale of growth. Anyone who believes there would be the space to do that at Heathrow, which already blights the lives of hundreds of thousands of Londoners, is quite simply crackers.”

The lands at Heathrow would not become “a tumbleweed-infested ghost town or a wasteland”, he said, since they could be dug up and developed into a 1930s garden city for people who were increasingly being priced out of the London housing market.

Mr Johnson’s plans will be lodged later this week with the British government-appointed airport commission headed by Sir Howard Davies, though it has been told not to report before the 2015 general election.

Thames estuary planThe Conservative mayor has backed away from an even more ambitious plan to build an airport in the Thames estuary, though his critics say he will favour anything that will not upset west London voters who do not want Heathrow to grow.

Mr Johnson said he had not abandoned his estuary plans – dubbed “Boris Island” by some – but said the Foster plans had the “greatest potential”. He has also mooted the possibility that Stansted could be turned into a four-runway airport.